feat: add GORM integration with repository pattern

- Add GORM models in models/ directory with proper column tags
- Create repository interfaces and implementations in core/repository/
- Add database package with MySQL and SQLite support
- Add UUID ID utility for GORM models
- Implement complete repository layer replacing SQL-based data access
- Add database migrations and index creation
- Support both MySQL and SQLite drivers with auto-migration

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/util/id/id.go

@@ -0,0 +1,48 @@
+package id
+
+import (
+	"encoding/binary"
+
+	"github.com/google/uuid"
+)
+
+// New creates a new binary ID (16 bytes) using UUID v4
+func New() []byte {
+	u := uuid.New()
+	return u[:]
+}
+
+// ToUUID converts a binary ID back to UUID
+func ToUUID(b []byte) (uuid.UUID, error) {
+	return uuid.FromBytes(b)
+}
+
+// String returns the string representation of a binary ID
+func String(b []byte) string {
+	u, err := uuid.FromBytes(b)
+	if err != nil {
+		return ""
+	}
+	return u.String()
+}
+
+// FromString parses a string UUID into binary format
+func FromString(s string) ([]byte, error) {
+	u, err := uuid.Parse(s)
+	if err != nil {
+		return nil, err
+	}
+	return u[:], nil
+}
+
+// Uint64ToBytes converts a uint64 to 8-byte slice
+func Uint64ToBytes(v uint64) []byte {
+	b := make([]byte, 8)
+	binary.BigEndian.PutUint64(b, v)
+	return b
+}
+
+// BytesToUint64 converts 8-byte slice to uint64
+func BytesToUint64(b []byte) uint64 {
+	return binary.BigEndian.Uint64(b)
+}